1. Field of the Invention
The present invention pertains to prestressed concrete members and in particular to structural concrete members which are prestressed using post-tensioning techniques.
2. Description of the Related Art
Prestressed concrete members are used extensively in modern construction practices. Prominent applications of prestressed concrete include bridges, building columns and liquid storage tanks. Common to each of these applications, is a goal of eliminating tension forces in the concrete members. Such tension loads in the concrete member have, in the past, been avoided by the use of either pre-tensioning or post-tensioning techniques. In this latter technique, the reinforcement members, such as those frequently comprised of steel tendons embedded within the concrete article, and are tensioned after the concrete hardens so as to place the concrete structure in compression.
U.S. Pat. Nos. 4,771,530, commonly assigned, discloses an improved method for the post-tensioning of concrete members wherein the concrete member is disposed within an outer rigid casing. A cementitious grout is then injected between the outer rigid casing and the outer surface of the cylindrical concrete member. The grout is allowed to harden to form a permanent prestressing pressure on the concrete article. Once the fluidically injected medium is hardened, the risk of loss of the medium from the assembly is eliminated. While finding ready commercial acceptance, certain improvements could be made to these prestressed articles. For example, resilient seals must be provided between the concrete article and the outer rigid casing, particularly at the ends thereof, to prevent loss of the prestressing medium which is fluidically injected between the casing and the concrete article. Resilient gaskets employed in the aforementioned prestressed articles are typically hidden from view behind the rigid casing. For resilient gaskets of fixed size, particularly a fixed cross-sectional size, the compressing pressure on the gaskets which is relied upon to form a seal against the pressure of the injected medium is controlled at the time of fabrication of the outer casing and cannot thereafter be easily adjusted as desired, in preparation for the grout injection.